The newer squash may not have been properly pollinated and are aborting if they are aborting when rather small (a few inches).

Are you giving them adequate N? Over-watering can also cause these issues if you believe your N levels are okay, especially if the vine itself is showing yellowing.

I'd start by holding back on watering so much unless you're sure this is something your plants are used to handling...you've obviously grown this before with success.

You have to be careful with N and squash if you do amend with it...they're low feeders of N (though high feeders of P/K).

I would agree with CRN, but leaves wilting is a major symptom, and is usually a sign of damage from either SVB or CB. It is supposed to be resistant to SVB, Have you checked if the stem is pristine, specially near the ground? If you cut off a leaf, will the sap string, like mucus when you have a cold? It could also be mice/gophers trimming the roots underground. Sun wilting is a sign of a disturbed circulatory system.

Depending upon where OP lives, could also be soil nematodes.

Thank you very much for your responses.
I checked the bases of the plants and they all look normal. I don't detect any holes, but I did see more ants than I thought there were originally. However, I had even more ants last year and they didn't kill anything. I also saw several tiny fresh leaves and flower formations at the base of the plants, which I find very weird. I cut off a semi-wilted leaf just now and the stem is hollow. The little bit of moisture I could squeeze from it was clear, not stringy or like mucus. I watered this morning but the soil three inches down seems quite dry already, and crumbly. I wonder if that's normal? Should I cut off the yellowing stems that are attached to the dying soft baby squash? Will the cuts promote even more distress and bacteria growth or might it help the plant recover and put out new stems? How do butternuts like fish emulsion as food? I was thinking of trying it. This is turning into a real learning experience for me and I hope I learn my lesson well because I don't care to repeat this class next year!
Thanks for helping me figure this out! :)

SAME thing happened to one of my butternut squash plants. Wilted leaves...looked like either SVB attack or cucumber beetle bacterial wilt. The other butternut squash plant was fine, the other squash plants (yellow and zucchini) were fine. So, I decided to pull it just in case there was a virus that could be transmitted to the other plants.

I also noticed ants. I did not notice SVB's or sticky substance from the stems when cut. Upon reflection, I think its roots were too shallow due to heavy rains when the seeds were planted. I think it was just too hot for the shallow stems. I probably could have buried it deeper (covering the stems) and watched it, but I chickened out fearing disease.

Planted new seeds which have sprouted. All the other plants are still fine. It has been raining for the past two days straight - THAT concerns me but there is nothing I can do about it. I will check the soil depth after the rain has stopped...trying to learn as I go. :-)

If you don't suspect disease, try burying it a bit deeper, perhaps putting fresh soil up around the lower stems. The link below is a previous thread that talks about burying stems (the subject is SVB but also recommends stem burying).

Here is a link that might be useful: Squash Vine Borer/Remove Plant?

Thanks, LKZZ! I checked out the link and it was interesting. It never really occurred to me that burying the stems would cause them to root. I guess I thought that it would cause them to rot. I am definitely going to add fresh soil at the base of the plant and see what happens next. In any case, I have tried various watering schedules and nothing has changed. All the newest baby squash are yellowing then turning black. This applies to any butternut fruit that is 4 inches and smaller. The leaves are turning yellow in the center where the stem attaches. When the plant wilts mid-day, I can clearly count 12 good sized squash and that's it for 4 plants, a paltry sum indeed. All the butternut plants are affected, but the zucchini and acorns are gorgeous and right next to the butternuts. Here in southern Cali, there is still plenty of time to plant more if I pull these. But since the only garden space I have left is where these 4 are, I would have to plant any new butternuts in the same place. Do you think I should cut off the yellowing part of the stems and bury the ends in the dirt, or should I bury them mid-stem?
P.S. One other thing I noticed is the scent and presence of cat poo. I don't think it's good for plants or people. I wonder if this could be the problem?

You could try hand pollination in case it's a pollination problem.

I thought I would post an update, just in case anyone else ever has this problem. Over the last month, I have fertilized the squash plants twice with E.B. Stone's Organic Vegetable Food, approx. 2 weeks apart. I water my garden by hand, and I remembered that last year I used to count to 10 while watering each plant. This year I had been counting to 40. Since I had stellar results last year, I went back to counting to 10. At this point, half the squash leaves are yellow and wilt in the sun and half are new, fresh, vibrant green and do not wilt in the sun. I am also finally getting some new baby butternuts that appear to be "taking." I have some real concerns, though, because I did not end up cutting off any yellowing stems, so the new green leaves are actually growing from the yellow stems. Therefore, I have to wonder about the quality of my squash, both the ones that have been growing from the beginning and the new little ones. Some of the ones that have weathered the storm actually appear nearly ready to pick (brownish stems), which is still a bit early for harvest, even though they have been on the vine for around 3 months. I am seriously wondering what these are going to taste like, they have been through so much this year. They are usually so luscious and hold up wonderfully in storage. We will see!

How well does your soil retain moisture? Does it stay moist several inches down for several days after you water?
How well does your soil drain?
How much organic matter is in your soil?
What is that soils pH?
Aside from the ants what other soil life do you see?
Where in the United States are you? Zone 10 only tells us what plants might survive a normal winter for wherever you are.

This post was edited by kimmsr on Thu, Jul 11, 13 at 6:06

Hi kimmsr,
I would say my soil does not hold moisture particularly well, even though we do amend it and add organic matter every year. On the other hand, it does appear to drain well. A 10"x10" hole I dug two days ago and filled with water was drained in less than 10 minutes.The native soil is clay. We live in a very dry climate east of San Diego, where we are lucky to see rain once during the summer. It did happen to rain just a few days ago, and I can already see that my entire garden loved it, including the squash, which as of today show 85% new growth. Yay!
As for the pH, I have no idea. I thnk it's time to invest in a soil test kit. In addition to ants, I see some roly-polies, some white flies, an ocassional pincher bug, and some weird tiny blackish bugs that emerged from the base of one of my Black Krim tomato plants that I pulled yesterday as it was doing poorly. I know I will have to spray all the plants with Neem at the bases now as I am sure these little bugs must have moved on to all my healthy tomato roots. I also saw them on my peach tree. My garden also shows evidence of visiting spiders. I really do not like to spray my garden with anything, but Neem has been quite gentle and I do want a harvest!
Yesterday I got brave and chopped off two long yellow stems from one of the squash plants. It meant sacrificing some new growth and one new baby squash but this is still a bit of an experiment, after all. I may trim again today and bury the end of one stem in the soil just to see what happens. The older squash is sort of a funny color, with yellowish tinge in spots instead of the overall nice deep tan color I am used to. I hope it still tastes good! :)